This invention relates to an exhaust gas particle sensor and in particular to a vehicle exhaust smoke sensor.
Increasingly legislation is being enacted to set limits on vehicle exhaust smoke, particularly from diesel engined vehicles.
It has been proposed in our co-pending patent application No. 9105731 that levels of smoke or other particles in vehicle exhausts could be sensed by measuring the opacity of the exhaust gases.
The opacity is measured by passing a light beam across a chamber through which the exhaust gases are flowing and measuring the reduction in the intensity of the light beam after its passage through the chamber.
For compression ignition engines, also known as diesel engines, the current UK exhaust smoke limit is defined as an absorption coefficient of 3.2 m.sup.-1.
In practice a number of problems have been encountered with this approach. Firstly, it is desirable to have the optical path length through the chamber of adequate length in order to obtain the optimum change in light beam intensity for a given opacity. However it has been found that where the size or shape of the chamber is different to that of the pick-up or inlet tube collecting exhaust gases the maximum measured opacity, or peak opacity, tends to be less than expected
Secondly, in order to prevent sooting up of the windows by deposition from the exhaust gases, clean air has been used to prevent the exhaust gases coming into contact with the windows through which the light beam enters and leaves the chamber. It has been found that the use of such air can produce erratic opacity measurements.
This invention was made in an attempt to produce an exhaust gas particle sensor at least partially overcoming these problems.